docs(rfc): define Handoff access control

[Teingi]

Which issue or RFC does this PR close?

Closes #1395.

Rationale for this change

PowerContext has a complete Handoff continuity model, but the current optional static Bearer authentication does not express per-user, per-Workstream, or exact-Handoff visibility. A receiver's Handoff authorization observation is also not an ACL.

This RFC defines a least-privilege Access Control boundary for transferring one exact Handoff from user A to user B while preserving the existing Handoff, Receipt, evidence, and Runtime domain semantics. It also defines a replaceable authorization-provider boundary so deployments can use the built-in store, Casbin, OpenFGA, or an AuthZEN-compatible PDP.

What changes are included in this PR?

• Add matching English and Chinese RFC documents for Handoff Access Control.
• Define trusted Principal, resource, action, role, Access Binding, and audit models.
• Separate one-off exact-Handoff receiver grants from long-term scope roles.
• Define bounded citation-manifest evidence access without exposing the complete scope.
• Define public Access API shapes and x-powercontext-access OpenAPI metadata.
• Define one Server PEP shared by HTTP, MCP, and the Dashboard.
• Define AuthorizationProvider and optional RelationshipWriter integration boundaries.
• Document built-in, Casbin, OpenFGA, AuthZEN, OPA, and Cerbos provider profiles.
• Specify failure behavior, safe listing and pagination, migration modes, implementation slices, and acceptance tests.

Are there any user-facing changes?

No released behavior changes are included. This PR adds an RFC design only. A later implementation PR would add public Access APIs, authorization configuration, 403 behavior, and Handoff receiver UX after separate review.

How was this change tested?

• make docs-test
• uv run prek run --files docs/en/rfcs/1396_handoff_access_control.md docs/zh/rfcs/1396_handoff_access_control.md
• git diff --cached --check

AI usage statement

OpenAI Codex (GPT-5) was used to inspect the existing Handoff, HTTP, MCP, and RFC contracts; draft the bilingual RFC; and run repository validation. The author directed the design scope and reviewed the resulting artifacts.